Acetylcholine-induced endothelium-dependent contractions in the SHR aorta: the Janus face of prostacyclin.

Abstract

In the spontaneously hypertensive rat (SHR) and aging Wistar-Kyoto rats (WKY), acetylcholine releases an endothelium-derived contracting factor (EDCF) produced by endothelial cyclooxygenase-1, which stimulates thromboxane A2 receptors (TP receptors) on vascular smooth muscle. The purpose of the present study was to identify this EDCF by measuring changes in isometric tension and the release of various prostaglandins by acetylcholine. In isolated aortic rings of SHR, U 46619, prostaglandin (PG) H2, PGF2alpha, PGE2, PGD2, prostacyclin (PGI2) and 8-isoprostane, all activate TP receptors of the vascular smooth muscle to produce a contraction (U 46619>>8-isoprostane=PGF2alpha=PGH2>PGE2=PGD2>PGI2). The contractions produced by PGH2 and PGI2 were fast and transient, mimicking endothelium-dependent contractions. PGI2 did not relax isolated aortic rings of WKY and SHR. Acetylcholine evoked the endothelium-dependent release of thromboxane A2, PGF2alpha, PGE2, PGI2 and most likely PGH2 (PGI2>>PGF2alpha>or=PGE2>TXA2>8-isoprostane, PGD2). Dazoxiben abolished the production of thromboxane A2, but did not influence the endothelium-dependent contractions to acetylcholine. The release of PGI2 was significantly larger in the aorta of SHR than in WKY, and the former was more sensitive to the contractile effect of PGI2 than the latter. The inhibition of PGI-synthase was associated with an increase in PGH2 spillover and the enhancement of acetylcholine-induced endothelium-dependent contractions. Thus, in the aorta of SHR and aging WKY, the endothelium-dependent contractions elicited by acetylcholine most likely involve the release of PGI2 with a concomitant contribution of PGH2.

Concentration–response curves to U 46619 and PGF2α (top left), 8-isoprostane and PGE2 (bottom left) PGI2 (top right) and PGH2 (bottom, right) in aortic rings without endothelium of WKY and SHR. Data are shown as mean±s.e.m. of at least four different experiments.

Basal and acetylcholine-dependent release of prostaglandins (PGI2, PGE2, thromboxane A2 and PGF2α) in aortic rings, with and without endothelium, of WKY and SHR. Data are shown as mean±s.e.m. of at least four different experiments. The * indicates a significant effect of acetylcholine and # a significant difference between WKY and SHR.

Effects of dazoxiben (10 μM) on the basal and acetylcholine-dependent release of thromboxane A2 (a), PGI2 (b) and PGE2 (c), as well as the acetylcholine-induced endothelium-dependent contraction in the aortic rings with endothelium of SHR (d). Contractile experiments were performed in the presence of L-NA (100 μM). Data are shown as mean±s.e.m. of at least five different experiments. The * indicates a significant effect of dazoxiben.

PGI2 release (left Y-axis) and acetylcholine-induced contraction (right Y-axis) in aortic rings with and without the endothelium of WKY (top) and SHR (bottom). Contractions were obtained in the presence of L-NA (100 μM). Data are shown as mean±s.e.m. of at least three different experiments.

Effects of the COX inhibitors indomethacin (5 μM, top left), valeryl salycylate (3 mM, top right) and NS 398 (1 μM, bottom left), on the basal and acetylcholine-dependent release of PGI2, as well as on the acetylcholine-induced endothelium-dependent contractions (bottom right) in rings with endothelium of SHR aortas. Data are shown as mean±s.e.m. of at least six different experiments. The * indicates a significant effect of an inhibitor.

Effects of U 51605 at 0.5 μM (left) and at 1, 3 and 10 μM (right) on basal and acetylcholine-stimulated release of PGI2 (top) and thromboxane A2 (bottom) in isolated aortic rings with endothelium of SHR. Data are shown as mean±s.e.m. of at least five different experiments. As the experiments involving the various concentrations of U 51605 were not contemporary, the data shown in the graphs situated on the right-hand side are expressed in percentage of the control acetylcholine response. The * indicates a significant effect of acetylcholine and # a significant effect of U 51605.

Effects of U 51605 at 0.5 μM (left) and at 1, 3 and 10 μM (right) on basal and acetylcholine-stimulated PGE2 (top) and PGF2α (bottom) release in aortic rings with endothelium of SHR. Data are shown as mean±s.e.m. of at least five different experiments. As the experiments involving the various concentrations of U 51605 were not contemporary, the data shown in the graphs situated on the right-hand side are expressed in percentage of the control acetylcholine response. The * indicates a significant effect of acetylcholine and # a significant effect of U 51605.